According to a conventional configuration of the internal combustion engine, a crankshaft is provided with one or more crank pins that are arranged offset with respect to the rotation axis of the crankshaft itself. As a result, when the crankshaft rotates, the crank pins are displaced along a circular trajectory.
Connecting rods are coupled to the crank pins to transform the rotation of the crankshaft into a linear reciprocating motion in a known manner, or, vice versa, to transform a linear reciprocating motion (e.g. the motion of a piston coupled to the connecting rod) to a rotation of the crankshaft. At the ends of the crank pin, there are generally two crank arms (one for each side), to drive the connecting rod keeping it in the correct axial position. Friction between the internal surface (i.e. the one coupled to the crank pin) of the crank arm and the connecting rod may damage the connecting rod itself by excessive wear.
In more detail, the crankshaft can be produced by forging. As known, at the end of the forging process, a thin portion of metal is left on the crankshaft where the two dies meet (known as forging parting line). In particular, a sharp or pointed portion (i.e. an edge) of metal is left on the crank arm at the crank pin. This sharp portion acts as a blade on the connecting rod, wearing its side faces out.
It is thus known to operate on the border of the internal surface (e.g. by chamfering or topping) to remove or cover such a sharp portion. These additional machining operations are expensive and time-consuming.